In general, polymer materials are used as an encapsulant material that covers a light source of a light emitting diode (LED). In recent years, the polymer encapsulant materials are dispersed with fluorescent material and are used in devices.
For example, Non-Patent Reference 1 describes that an element composed of nitride fluorescent material dispersed in an epoxy resin can be used as an LED having a warm white color.
However, defective deterioration and deformation occasionally occurred in the above-described polymer encapsulant materials due to heat generated by emission of light from the LED light source. The brightness of the LED light source has been enhanced recently. As a result, heat generation has increased, and the problems of insufficient heat endurance of the polymer encapsulant materials has aggravated. On the other hand, non-patent reference 2 describes that the fluorescent materials made of oxynitride and nitride have higher heat endurance compared to oxide fluorescent materials. Therefore, there is an increasing demand for encapsulant materials of still high endurance.
Based on the above-described circumstance, glass materials having relatively high endurance has been investigated as an encapsulant material that can replace the polymer materials.
For example, Patent References 1 and 2 are related to fused glass that seals a light emitting diode. Patent Reference 3 describes a glass formed by a fusion method for dispersing fluorescent material therein, and a method for manufacturing a fluorescent material-dispersed glass by dispersing the fluorescent material in a fused glass.
However, the fused glass requires a reaction process at very high temperature. Therefore, there has been a problem of dagradation of the fluorescent material due to reaction of the fluorescent material with the fused glass during the high temperature reaction process.
For example, Patent Reference 2 describes that the fluorescent material is degradated at a temperature exceeding 900° C. where the fused glass used in the process is mainly composed of TeO2.
Sol-gel method does not require a high temperature reaction process. Therefore, according to the sol-gel method, it is possible to produce a fluorescent material-dispersed glass while suppressing the dagradation of the fluorescent material by avoiding reaction of the fluorescent material with the glass.
For example, Patent Reference 4 describes a fluorescent material-dispersed glass produced by dispersing the fluorescent material in the glass formed by the sol-gel method. Patent Reference 5 describes a fluorescent material-dispersed glass produced by dispersing nano-particles in the glass formed by the sol-gel method.
However, according to the conventional sol-gel method, it is difficult to form a bulk glass, and only a silica glass with low-refractive index can be obtained. Patent Reference 4 also utilizes silica glass and describes that a light of constant color tone can be emitted by controlling the number and size of bubbles. These effects can be obtained by changing the difference of refractive index between the fluorescent material and the glass. For example, if a glass added with TeO2 having high refractive index is obtained, it is possible to control the refractive index of the glass to be similar to that of the fluorescent material, thereby suppressing reduction of emission intensity in the interface between the fluorescent material and glass, and improving the emission efficiency.
Patent Reference 6 describes a fluorescent material-dispersed glass produced by mixing a glass powder and a fluorescent material powder and sintering the mixture. However, the above-described problems could not be solved even when this glass was used.